Thousands of computer users may lose Internet access on Monday, when the deadline for a temporary fix to a malicious software scam shut down by the FBI last year expires.

Millions of computers were infected with the so-called “Internet Doomsday” virus used in the hacking scam, which redirected Internet searches through DNS servers used by the scammers. (Who, in turn, allegedly netted $14 million in bogus advertising revenue.) After U.S. and Estonian authorities busted the malware ring last November, a federal judge ordered that the FBI use temporary servers while the malware victims’ PCs were repaired. The temporary servers will shut down at 12:01 a.m. EDT on Monday, meaning anyone using a computer still infected with the virus will likely lose Internet access.

“Connectivity will be lost to the Internet PERIOD,” Symantec, the online security firm, said in a blog post. “If your computer is still using DNS entries that are pointing to the FBI servers on July 9, you will lose TOTAL access to the Internet. No connecting to the office from home, no updating Facebook, nothing until the DNS settings are fixed.”

It’s unclear how widespread the “blackout” will be. According to a working group set up by security experts, more than 300,000 computers remained infected as of June 11, including 69,000 in the United States. Last week, 245,000 computers were said to be still infected with the so-called Alureon virus, according online security firm Deteque, including 45,355 U.S. machines.

According to Reuters, U.S. Internet providers including AT&T and Time Warner Cable “have made temporary arrangements so that their customers will be able to access the Internet using the address of the rogue DNS servers.” And the problem, security experts say, is relatively easy to fix.

Have you been affected with the Internet Blackout? What tools did you use to diagnose your PC of the “Internet Doomsday” virus?

Notorious hacker group Anonymous has previously stated its intentions to shut down the Internet on Saturday, March 31st, as a form of protest.

“To protest SOPA, Wallstreet, our irresponsible leaders and the beloved bankers who are starving the world for their own selfish needs out of sheer sadistic fun, on March 31, anonymous will shut the Internet down,” the group stated last month. “Remember, this is a protest, we are not trying to ‘kill’ the Internet we are only temporarily shutting it down where it hurts the most.”

Operation Global Blackout 2012 looks to shut down the Internet by disabling its core DNS servers, thus making websites inaccessible. Cyber security experts claim that it is unlikely that such an attack would be effective, however, and there is really no need to fear. Read on for more.

“The Anonymous hackers can certain cause local pockets of disruption, but these disruptions are going to be localized to networks where their attack machines are located, or where their ‘reflectors’ are located”, said Robert Graham of Errata Security. “It’s unlikely they could take all of them down, at least for any period of time. On the day of their planned Global Blackout, it’s doubtful many people would notice.”

Graham does issue a cautionary note, however. “Just because I say Anonymous can’t do it doesn’t it mean it can’t be done,” the security expert said on Errata’s blog.